Retinoids (vitamin A-derivatives) have important physiological functions in a variety of biological processes. During embryonic growth and development, as well as during growth and differentiation of adult organisms, retinoids act as hormones and participate in the regulation of gene expression in a number of cell types. See Lied et al. Trends Genet., 17:427-433 (1992). It is believed that the effects of these molecules are mediated through two classes of nuclear ligand-controlled transcription factors, the retinoic acid receptors (RARs) and the retinoid X receptors (RXRs), Benbrook et al., Nature, 333:669-672 (1988); Brand et al., Nature, 332:850-853 (1988); Giguere et al., Nature, 330:624-629 (1987); Mangelsdorf et al., Nature, 345:224-229 (1990); Mangelsdorf, et al. Genes Dev. 6:329-344 (1992); Petkovich et al. Nature 330:440-450 (1987); and Zelent et al., Nature 339:714-717 (1989).
Apart from their role as hormones in cellular growth and differentiation, retinoids are also involved in the visual process as the stereo isomer 11-cis retinaldehyde of retinaldehyde is the chromophore of the visual pigments. See, e.g. Bridges, The Retinoids, Vol. 2, pp 125-176, Academic Press, Orlando, Fla., (1984).
Under normal physiological conditions most cells, both ocular and non-ocular, obtain all-trans retinol as their major source of retinoids. Despite the many different metabolic events taking place in different tissues, it is known that a common extracellular transport machinery for retinol has evolved. Specifically, in plasma, retinol is transported by plasma retinol binding protein (RBP). See Goodman et al., The Retinoids, Academic Press, Orlando Fla., Volume 2, pp. 41-88 (1984). The active derivatives of retinol, i.e., retinoic acid in non-ocular tissues and mostly 11-cis retinaldehyde for ocular tissues, are generated by cellular conversion of trans-retinol, using specific mechanisms. To date, none of these mechanisms have been fully defined at the molecular level and several of the enzymes involved have only been identified by enzymatic activities. See Lion et al., Biochem. Biophys. Acta. 384:283-292 (1975); Zimmermann et al., Exp. Eye Res. 21:325-332 (1975); Zimmermann, Exp. Eye Res. 23:159-164 (1976) and Posch et al., Biochemistry 30:6224-6230 (1991).
Polarized retinal pigment epithelial cells (polarized RPE) are unique with regard to retinoid uptake since all-trans retinol enters these cells via two different mechanisms. Retinol accumulated from RBP is taken up through the basolateral plasma membrane, while all-trans retinol, presumably taken up from the interstitial retinol-binding protein (IRBP) following bleaching of the visual pigments, may enter through the apical plasma membrane. See Bok et al., Exp. Eye Res. 22:395-402 (1976); Alder et al., Biochem. Biophys. Res. Commun. 108:1601-1608 (1982); Lai et al., Nature 298:848-849 (1982); and Inu et al., Vision Res. 22:1457-1468 (1982).
The transfer of retinol from RBP to cells is not fully understood. In a number of cell types, including RPE, specific membrane receptors for RBP have been identified. This is consistent with a receptor-mediated uptake mechanism for retinol. For example, isolated retinol binding protein receptors, nucleic acid molecules coding for these receptors and antibodies binding to the receptor have been taught, in references relating to the first of the two mechanisms. See Bavik et al., J. Biol. Chem. 266:14978-14985 (1991); Bavik, et al. J. Biol. Chem. 267:23035-23042 1992; Bavik et al., J. Biol. Chem. 267:20540-20546 (1993); and copending U.S. application Ser. No. 083,539 and International Publication WO 93/23538, all of which are incorporated by reference herein. See also Heller, J. Biol. Chem. 250:3613-3619 (1975); and Bok et al., Exp. Eye Res. 22:395-402 (1976).
Retinol uptake on the apical side of the RPE for the regeneration of 11-cis retinaldehyde, is less well characterized. Regardless of the origin of all-trans retinol, however, the synthesis and apical secretion of 11-cis retinaldehyde seems to be the major pathway for accumulated retinol in the RPE. At present, it is not known whether similar mechanisms are used with regard to cellular retinol uptake through the basolateral and the apical plasma membranes. Available data do show that functional receptors for RBP are exclusively expressed on the basolateral plasma membrane of RPE-cells. Bok et al., Exp. Eye Res. 22:395-402 (1976).
It is also known that RPEs express a 63 kDa protein (p63). This molecular weight, and all others, is by reference to SDS-PAGE, unless stated otherwise. It has also been shown by chemical cross-linking that this protein may be part of an oligomeric protein complex which functions as a membrane receptor for plasma retinol-binding protein (RBP) in RPEs, or a component of the retinoid uptake machinery in RPE cells. See Bavik et al, J. Biol. Chem. 266:14978-14875 (1991); Bavik et al., J. Biol, Chem. 267:23035-23042 (1992), U.S. patent application Ser. No. 083,539 and PCT application WO93/23538. The p63 protein has been isolated and the corresponding cDNA cloned. See Bavik et al., J. Biol. Chem. 267:20540-20546 (1993). Nothing in these references suggests the existence of the protein which is a feature of this invention.